Documentation for yab templates in README

[HelloGrayson]

Hoping to write my first yab template, but couldn't find any docs ;(

[prashantv]

There's some information in the man page if you look for "templates":
http://yarpc.github.io/yab/man.html

[kriskowal]

Changed issue name to track documentation in README. The topic is covered in the man page, but we should have this in the README for discovery.